Liquid crystal displays of the state of the art often show a reduced brightness due to light absorption in the optical path, which is in particular caused by the linear polarizers that are used in displays. For example, in backlit displays these linear polarizers can absorb more than 60% of the light intensity emitted from the backlight.
Therefore, circular reflective polarizers--in the following being referred to simply as "reflective polarizers"--have been developed that can very efficiently convert unpolarized light into polarized light. They usually comprise a layer of a chiral liquid crystalline material, like e.g. a cholesteric liquid crystal, that exhibits a helically twisted molecular structure and further exhibits planar alignment, i.e. wherein the axes of the molecular helices are oriented substantially perpendicular to the plane of the layer.
If unpolarized light is incident on such a reflective polarizer, 50% of the light intensity are reflected as circularly polarized light with the same twist sense as that of the molecular helix, whereas the other 50% are transmitted. The reflected light is depolarized (or its sense of polarization is reversed) in the backlight of the display, and is redirected onto the polarizer. In this manner theoretically 100% of a given waveband of the unpolarized light incident on the reflective polarizer can be converted into circularly polarized light.
The circularly polarized light can be converted into linear polarized light by means of a quarter wave optical retarder and optionally also a compensation film.
The bandwidth .DELTA..lambda. of the waveband reflected by a reflective polarizer as described above depends on the birefringence of the mesogenic material .DELTA.n and the pitch of the molecular helix p according to the equation .DELTA..lambda.=.DELTA.n.times.p. Thus, the bandwidth is limited by the birefringence of the material. However, for an application in liquid crystal displays it is desirable that the bandwidth of the polarizer should comprise a substantial portion of the visible wavelength range.
Recently reflective polarizers have been developed that reflect a wider waveband of incident light. These polarizers comprise a liquid crystalline material with a helically twisted structure and a planar orientation, and are further characterized in that the pitch of the molecular helix varies in a direction normal to the layer, which leads to a large bandwidth of the reflected wavelength band.
The European Patent Application EP 0 606 940 discloses a circular reflective polarizer with a bandwidth of up to 400 nm that is consisting of a film of a polymerized cholesteric liquid crystal.
A suitable method to prepare a broadband reflective polarizer is e.g. by coating of a polymerizable liquid crystalline material with a cholesteric phase on a substrate or between two substrates in form of a thin layer, aligning the material into a planar orientation and polymerizing the material to freeze in the helically twisted, planar liquid crystalline phase structure.
Such a broadband reflective polarizer, when being used in combination with a quarter wave foil and optionally also a compensation film in a liquid crystal display, can give an increased brightness of up to 60-70% at normal viewing incidence.
At larger viewing angles, however, the luminance of a display with a broadband reflective polarizer decreases, and normally falls below the luminance of a display with a conventional linear polarizer (usually a dichroic polarizer) at some angle within a 60.degree. viewing cone. The angle at which this occurs in the horizontal viewing plane is called the cross-over angle. For a broadband cholesteric polarizer with a high brightness gain this usually occurs at a maximum of around 50.degree. when a compensator film is used.
Furthermore, at large viewing angles often an undesired color shift of the light transmitted by such a broadband reflective polarizer is observed.